Depends on the ray. Our rays feel like they are starving not long after they just get done eating what seems like a feast and a half. We feed them huge amounts before we go to bed at night, and they are going crazy for food as soon as we wake up in the morning. So, in that case like that, a starvation period could be from a couple of days to a week.
Like turkey boy said, make sure the ray is completely healthy first.
And I'm not really sure about the red-belly when you have a bare-bottom tank with those water parameters. I'll be keeping an eye out for what the more experienced suggest.